For a natural gas engine having a three-way catalyst system for the engine exhaust, the air/fuel ratio must be held within a very narrow window to obtain simultaneous high efficiency of NOx reduction and CO oxidation in the catalyst. The disadvantage of this strategy is the difficulty of achieving the accuracy of air/fuel ratio control needed to obtain simultaneous high efficiency of both reactions, and maintaining this accuracy over the lifetime of the engine-catalyst system. A particular problem encountered is the tendency of the catalyst to age, resulting in further narrowing of the required control window, and also shifting its center.
In an attempt to overcome this control issue, some engine manufacturers have taken an alternate strategy to achieve high efficiency of NOx and CO oxidation in the catalyst. This alternate strategy divides the catalyst into two beds and operates the NOx reduction reaction in the first bed using a wider, less critical window of air to fuel ratio control. Secondary air is then added to the products exiting the first catalyst bed, prior to their entering the second catalyst bed. This secondary air supply ensures that there is sufficient oxygen present to effectively oxidize the remaining CO in the feedgas entering the second catalyst bed from the first catalyst bed.
Unfortunately, while such systems have been employed on certain passenger car engines under certain conditions, this type of system requires the inclusion of an engine driven air pump specifically fitted for the purpose and active controls to enable and disable and/or modulate the air delivery process. As should be apparent, such a system is disadvantageously expensive and complex owing to the addition of the air pump and active controls.
As such, there is a need in the art for a secondary air delivery system that can provide the needed oxygen to the second bed of a two bed catalyst to ensure effective oxidization of CO and unburned HC without unduly increasing the cost and complexity of the system. Embodiments of the present invention provide such a system and method.